(1) Field of the Invention
The present invention relates to a metal-ceramic composite article and a process for manufacturing the same. More specifically, the invention relates to a metal-ceramic composite article in which a metallic member and a ceramic member are joined together through fitting.
(2) Description of the Prior Art
Since ceramics are not only hard and excellent in wear resistance, but also excellent in mechanical characteristics and corrosion resistance at high temperature, they are suitable for a structural material for rotors of a gas turbine and a turbocharger which require mechanical strength and wear resistance at a high temperature.
For this reason, it has been investigated that the gas turbine rotor and the turbocharger rotor are made of ceramics. For instance, U.S. Pat. No. 4,396,445 discloses a turbine rotor having the structure that blade portions and a shaft are made of ceramics. According to the turbine rotor of this structure, a threaded portion is formed at one end of the ceramic shaft, and metallic impellers of a compressor are attached to the threaded portion. However, such a turbine has the drawback that the threaded portion of the ceramic shaft is broken due to the difference in the thermal expansion between the metallic material constituting the impellers of the compressor and the ceramic material constituting the shaft portion when the turbine rotor is in use. Further, it also has the drawback that the screw thread cutting of the ceramic requires a high precision technique, which requires time and cost.
As a countermeasure against such drawbacks, there is disclosed in Japanese Utility Model Registration Application Laid-Open No. 92,097/1982 a structure in which a cylindrical portion formed at an end of a metallic shaft is fitted with a ceramic shaft of a turbine rotor. However, this structure has the disadvantage that when the ceramic shaft is fitted into the metallic shaft after the outer surface of the cylindrical portion of the metallic shaft is surface-hardened to increase the wear resistance of a bearing-contact surface of the metallic shaft, cracks are produced at the surface-hardened portion of the metallic shaft. Moreover, when the surface of the metallic shaft is subjected to the surface hardening treatment, such as a nitriding treatment, after the metallic shaft and the ceramic shaft are fitted together, the tightness at the fitting portion decreases or the fitted metallic shaft slips out from the fitting portion. In addition, when quenching is carried out after the metallic shaft and the ceramic shaft are fitted together, the ceramic shaft slips out from the fitted portion of the metallic shaft due to the phase transformation of the metallic shaft due to the quenching. Therefore, the above structure has the drawback that since the wear resistance is poor at the bearing-contact surface of the metallic shaft, it can not be practically used.